Abstract
Viscosity modifying agent (VMA) is added to mortar to increase water retention so that more rendering plaster and tile can be installed. But, VMA would increase viscosity of pore solution to slow down water movement, and thus a dry shell may form due to evaporation. Consequently, Vicat needle test seems improper to determine set times of VMA modified mortars. The ultrasonic pulse velocities (UPV) of early-age VMA modified paste (VMA paste) are compared with Vicat test results in this paper. UPV were found to be clearly depicting the dosage effect of VMA on cement retardation, whilst Vicat test could not. Without adding VMA, initial set of Vicat test was found to be near the timing of UPV at 1420 m/s and final set was found no clear relationship with UPV values. However, with the addition of VMA, more investigations by UPV are needed before new criteria on determining both initial and final set times can be proposed.
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